1. Field of the Invention
The disclosure relates to printing structures made of polymer nanobeads. More specifically, the disclosure relates to a method and apparatus for providing printed structures of polymer nanobeads that have substantially flat printed surfaces and provide improved optical properties.
2. Description of Related Art
Microelectromechanical systems (MEMS) fabrication falls into two categories: bulk and surface micromachining. The former consists of removing significant portions of the underlying substrate to define the desired features, while the latter consists of addition and subsequent removal of materials on top of the base substrate. Both categories rely on semiconductor industry manufacturing methods, especially lithography, a technique which relies on mask layers to selectively protect the covered portions while etching the exposed regions. The price of MEMS remains significantly larger than its semiconductor counterparts. The semiconductor model is based on everything using standardized processes and packaging to outweigh and spread the large overhead cost over many diverse products. MEMS typically require specialized processes and packaging for each product and cannot recover the large overhead costs without being significantly more expensive. As such, novel fabrication techniques need to be explored to lower the price of MEMS devices.
There has been a significant effort in lowering the cost of MEMS, from investigating low-cost materials to highly efficient fabrication processes. One category is the development of tools and techniques that would eliminate the need for standard photolithography tools. Inkjet is an alternative to standard lithography processes and can be used in a drop-on-demand (“DOD”) fashion to deposit multiple small droplets of desired materials at digitally-defined positions. These defined positions can pattern features for several purposes, including integrated circuits and MEMS devices. The mask-less concept has several advantages including less material waste, fewer design-to-fabrication delays, and faster fabrication.
Poly(methyl methacrylate) (PMMA) or poly(methyl 2-methylpropenoate) is a thermoplastic material with transparent color. PMMA is the synthetic polymer of methyl methacrylate. PMMA's common names include acrylic glass, acrylic or Plexiglas. Acrylic, or acrylic fiber, can also refer to polymers or copolymers containing polyacrylonitrile. PMMA has been used as an alternative to glass and in competition with polycarbonate (PC) plastic. PMMA is often preferred over PC because of its moderate properties, easy handling and processing, and low cost. PMMA is more brittle than PC (but less brittle than glass) when loaded or when under an impact force. PMMA is conventionally produced by emulsion polymerization, solution polymerization and bulk polymerization.
PMMA polymers with unique optical and thermoplastic characteristics have been used as waveguide materials in optoelectronic devices or as sacrificial spacer layers in micro-electro-mechanical systems (MEMS) where smooth surface morphology is essential. Although simple methods of depositing PMMA films from solution have already existed, including for example, spin coating, dip coating or spray coating, lithographic techniques are most commonly applied. However, lithographic techniques are expensive and time-consuming. A fast, low-cost, patterning technique such as inkjet printing would be of great interest for the preparation of planar PMMA features over large area.
Inkjet printing technology has already been adopted to deposit a variety of polymer inks for digital fabrication (“DigiFab”) applications. Non-uniform polymer patterns are often observed as a result of inkjet printing. The non-uniform patterns have been attributed to the complicated ink wetting and drying process on ink-impermeable substrate surface.
To improve the surface morphology of the printed PMMA films, research efforts have focused on ink formulation or surface treatment. For example, conventional ink formulations have been devised by mixing volatile and slow-drying solvents as the ink vehicle (or carrier). Other conventional approaches have modified the substrate surface to have ink-attracting/repelling regions so as to engineer surface tension. The conventional approaches have failed to produce a substantially flat printed PMMA surface suitable for high-tech applications. The PMMA layer produced with conventional method also lacks the desired optical properties. Therefore, there is a need for a method and apparatus for constructing polymer structures with improved morphological and optical properties.